1. The Field of the Invention
The present invention is related to a hybrid composite/metal pressure vessel. More particularly, the present invention is related to a novel pressure vessel configuration capable of satisfying Insensitive Munitions requirements.
2. Technical Background
Historically, rocket motor cases have generally been made of metal. Metal cases have traditionally been favored because of their inherent toughness and versatility. A variety of end closures and fin attachments can be easily mounted to a metal case, making metal cases suitable for many applications. Also, metal cases can be quickly and inexpensively manufactured. Thus, metal cases have generally provided excellent performance and versatility at an acceptable cost.
However, metal cases have the potential of containing substantial internal pressure when hot or damaged. Thus, if a rocket motor having a metal case is inadvertently ignited, either through exposure to high temperatures or through bullet or fragment impact, the combustible propellant may react violently under the contained pressure. Ignition pressure may escape through the rocket nozzle, generating substantial thrust, propelling the burning rocket motor. Additionally, the propellant may explode, projecting fragments of the metal shell in many directions. The dangers to personnel and equipment resulting from the inadvertent ignition of rocket motors having a metal case are obvious.
As a result of these dangers, tests to determine the sensitivity of rocket motors to conditions which may cause unplanned ignition have been developed. Munitions successfully passing such tests are generally categorized as "Insensitive Munitions." Thus, Insensitive Munitions tests attempt to measure the sensitivity of a rocket motor to cookoff, and bullet and fragment impact.
In response to the desire for an improved rocket motor case which can satisfy Insensitive Munitions requirements, a variety of "hybrid" metal/composite rocket motor cases have been developed. Such hybrid cases include a metal shell and a composite layer which can be affixed either inside or outside the metal shell. The metal shell typically is designed to provide substantial axial strength while the composite layer provides the hoop strength.
When such hybrid cases are subjected to cookoff or fragment tests, the composite layer is destroyed. Because the metal shell is incapable of supporting the hoop loads imposed as a result of the combustion pressure of the propellant, the shell will also fail, thereby eliminating the ability of the case to contain pressure, permitting the propellant to burn mildly. By reducing the pressure containment capability of the case, the violence with which the propellant may combust is reduced.
One proposed hybrid case design involves configuring small slots in the metal shell which will propagate when subjected to a predetermined pressure. The size of these slots is determined by calculating the "critical flaw size" of the metal shell at a given pressure. The slots which are configured in the metal shell are sized to be equal to the critical flaw size for a predetermined pressure.
Consequently, when the motor is subjected to an Insensitive Munitions event which ignites the propellant and causes the composite layer to fail, the full combustion pressure is imposed upon the metal shell. The slots in the metal shell, being of critical length for that pressure, immediately propagate.
One disadvantage with this design is that a substantial pressure may be required to induce propagation of the slots in the metal shell. Many propellants will react quite violently under minimal pressure containment. Indeed, the pressures required to generate propagation in slotted hybrid pressure vessels may be sufficiently high that unacceptably violent rupture occurs.
Another difficulty associated with the slotted hybrid pressure vessels arises when the metal shell is made of a high-strength steel. Because of the low fracture toughness of high-strength steel, the critical flaw size is quite small. Configuring slots in a metal shell made of high-strength steel cannot be done with conventional manufacturing methods. Hence, it is extremely expensive, if not impossible, to implement high-strength steels in a hybrid pressure vessel.
A further disadvantage associated with the use of slotted hybrid pressure vessels is their inability to take advantage of the full load-bearing capabilities of the composite layer. As the load on the composite layer increases, the composite layer deforms proportionally in response to the load increase. Typical composites can be subjected to one to three percent strain before failing.
However, when attached to a metal shell, the hoop strain in the composite layer is limited to the strain of the metal shell. As strain increases in the composite layer, the load is transferred through the bond to the metal shell. If sufficient hoop load is transferred to the metal shell, the slots in the shell could propagate, thereby destroying the metal shell's ability to bear axial load. Thus, the hybrid pressure vessel must be designed so that the maximum internal pressure will not cause the composite layer to strain significantly beyond the capability of the metal shell.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide an improved hybrid pressure vessel design which significantly reduces or eliminates the pressure containment capability of the metal shell when subjected to an Insensitive Munitions event.
It would be a further advancement in the art to provide a hybrid pressure vessel design which employs a metal shell made of high-strength steel.
It would be an additional advancement in the art if such a design would permit the composite layer to strain beyond the yield point of the metal case without destroying the axial strength of the metal case.
Such a hybrid pressure vessel and method for manufacturing is disclosed and claimed herein.